skate-3-rust-engine/docs/vehicle-sdk.md

Vehicle SDK v1

skate-vehicles pins Rapier 3D 0.35.3 and owns an independent, fixed-step rigid-body world. The existing skating solver remains authoritative outside vehicles. Rapier uses the installed map's collision triangles and resolves chassis collisions with the map and other vehicles. Wheels use Rapier raycast suspension. Each host tick is split into substeps of approximately 1/120 second or smaller.

First mod: Mario Kart

Run PLAY-MARIO-KART.bat, enable Mario Kart in Mods, resume and press F10. Walk/skate within four metres and press E to enter. W/S accelerate/reverse, A/D steer, Space brakes, left Shift handbrakes, R / right-stick click rights/resets. E exits once speed is below 3 m/s. F10 replaces a parked kart or resets an occupied kart. Live engine, speed, brake, steering and grip controls appear in its own draggable, resizable, scrollable mod window.

Controller: Y enter/exit, RT/LT accelerate/reverse, left stick steer, A brake, B handbrake, right-stick click reset while driving. The reset bind can be changed to left-stick click in the mod settings. F10 remains the keyboard spawn shortcut. Release controls before switching between driving and skating.

mods/mario-kart/kart.glb is generated from the user-supplied ZIP. It is excluded from Git. The importer preserves meshes/materials/embedded textures, normalizes the model to metres and creates named wheel pivots. To regenerate:

python tools/prepare_mario_kart.py C:/Users/Daddy/Downloads/kart-de-mario.zip mods/mario-kart

NumPy is required by the importer. Run Build-VehicleSDK.ps1 to build/stage the executable and original mod sources. The helper preserves existing modified mod files; copy updates manually when it reports a preserved file. PLAY-MARIO-KART.bat loads the top-level project mods/ directory via SKATE3_MODS. A standalone executable defaults to its adjacent mods directory. No model or game assets are committed.

Lua API

All keys belong to the calling mod. Commands cannot control another mod's vehicle. The host allows eight vehicles per mod and 32 total. Definitions are validated before allocation. Commands from failed Lua callbacks are discarded. Disabling, removing or reloading a mod removes its vehicles and releases its driver.

sdk.vehicle.spawn('kart', 'vehicle.json', {x, y, z}, heading_radians)
sdk.vehicle.enter('kart')
sdk.vehicle.control('kart', {
    throttle = 1,       -- -1 reverse through +1 forward
    steering = 0.25,    -- -1 right through +1 left
    brake = 0,         -- 0..1
    handbrake = false,
})
sdk.vehicle.tune('kart', {engine_force=1800, max_speed=25, tire_grip=3})
local car = sdk.vehicle.read('kart')
sdk.vehicle.exit('kart')
sdk.vehicle.reset('kart', {x, y, z}, heading_radians)
sdk.vehicle.remove('kart')

read returns nil for an absent vehicle, otherwise position, quaternion rotation (x/y/z/w), heading, signed speed in m/s, occupied, ready and phase. Phases are parked, entering, driving, exiting. Snapshots update at host callback boundaries; a spawn command is visible on a later callback. input() returns normalized keyboard/controller throttle, steering, brake, handbrake, interact and pad_buttons. Do not pass the full input table to control; copy only its four control fields. control must be refreshed; after 0.25 seconds without a command, throttle clears and a parking brake is applied. Controls are ignored during enter/exit animations.

tune supports optional engine_force (0..100000 N), max_speed (1..100 m/s), engine_volume (0..1), brake_impulse (0..10000), steering_angle (0.01..1.2 radians), tire_grip (0.1..20). Maximum speed limits engine application; it is not an absolute downhill speed cap.

Events passed to on_event: vehicle_spawned, vehicle_entering, vehicle_entered, vehicle_exited, vehicle_exit_blocked, vehicle_reset, vehicle_removed. Each includes owner and key. Map changes use the existing world_changed event; spawn new vehicles in the new world as needed. Invalid assets/commands appear as mod errors; they retire the mod's vehicles instead of crashing the game.

Vehicle definition

See mods/mario-kart/vehicle.json for a complete working definition. Coordinates are metres, +Y up, +Z forward, +X driver-left, with heading in radians around +Y. half_extents describes the outer chassis collision bounds. mass is kilograms. model_scale, model_offset and model_yaw affect the model only, not its collider.

Each wheel defines a chassis-local suspension mounting position, radius, steering/driven flags and optional unique model node name. Support is 2..8 wheels with at least one driven wheel. Wheel nodes should have local +Y steering and +X axle axes. Pivots must be correctly positioned; the host adds suspension travel, steering and spin to their rest transforms. The supplied kart importer does this.

Seat and exit are chassis-local offsets. Exit checks the nearby fixed ground and standing clearance; if unavailable, it returns the rider to the saved entry spot. The camera follows using camera_distance and camera_height. Model GLBs must embed all textures and buffers; filesystem/network references inside GLBs are rejected. A package may total 64 MiB; a model is limited to 32 MiB and an animation file to 16 MiB. Paths stay within the owning mod folder.

Rider animation support

The local kart package supplies fitted clips. Other definitions default their animation slots to null. Without a matching clip, the skater is hidden and entering/exiting completes immediately. This avoids showing an unrelated standing/skating pose on the kart.

Set animations.file to a package-relative JSON file and set slot names:

"animations": {
  "file": "rider.json",
  "enter": "get_in",
  "exit": "get_out",
  "drive": "drive_loop",
  "idle": "seated_idle",
  "reverse": "look_back",
  "brake": "braking",
  "steer_left": "turn_left",
  "steer_right": "turn_right"
}

The file is:

{
  "version": 1,
  "bone_names": ["EXACT_NAMES_FROM_sdk.animation.info().bone_names"],
  "clips": {
    "drive_loop": {
      "fps": 30,
      "frames": [["ONE_16_NUMBER_MATRIX_PER_BONE"]]
    }
  }
}

The strings in the example frames/bone list are explanatory placeholders. Actual frames must contain one 16-number, column-major, native model-space matrix per bone, in the exact native skeleton order. Matrices use metres and +Y up, matching the existing authored animation convention. They are full poses, not additive deltas. Obtain names from sdk.animation.info(); do not invent bone names. The renderer performs the existing native-to-GLB skin-basis conversion. Author the pose relative to the seat offset; enter/exit root motion is visual and does not move the Rapier chassis. The native skater is restored through its teleport path when exiting. Files with a mismatched skeleton, invalid matrices or missing named clips are rejected. Maximum 32 clips, 3600 frames per clip, 1..120 fps.

Enter/exit clips play once and their frame count/fps determines transition duration. Driving slots loop; brake and reverse select their respective base clips, with drive as fallback. Steering continuously blends that base towards the left/right pose. Vehicle phase changes blend with native bone-local interpolation; brake/reverse base-clip changes currently cut, so author compatible seated poses. Edit vehicle.json or rider.json and reload the mod to install new clips.

Current boundaries

The native skating simulation is suspended while driving, and normal skating resumes on exit. Vehicle time pauses with Escape and during replay. Replay entry and session-marker controls are blocked while driving. Vehicle motion is not recorded in skating replays. Native skater-versus-vehicle impact forces are not bridged yet: vehicles collide with the map and one another, but parked vehicles are not native skating obstacles. This API does not include weapons, damage, network synchronization or a racing ruleset.

Verification uses headless Rapier tests and window-free Lua tests. The game is not launched automatically; rendering, entry/exit and handling need manual playtesting.

Rider transitions and steering

The host blends vehicle phase changes over 0.4 seconds and the return to vanilla over 0.5 seconds, using the game's bone-local translation/scale lerp and quaternion slerp. Steering input is smoothed and blends the current driving pose towards steer_left or steer_right continuously, so these slots should contain compatible seated poses. Vehicle camera hand-offs ease over 0.5 seconds. While driving, chassis, wheels, rider and camera share the same interpolated fixed-step motion sample, avoiding relative jitter from separate camera damping. Resets discard the old motion sample. The board root is scaled away during vehicle playback and restored by the vanilla blend. The local Mario Kart package supplies fitted entry/exit, seated and steering clips; its proprietary rider.json is generated locally and is not included in source control.

Ramp clearance and mass distribution

These are shared definition fields, available to any mod, not Mario-specific code:

Field	Meaning	Default
collider_offset	Chassis-local collision centre, metres	[0,0,0]
collider_rounding	Rounded edge radius; below 95% of smallest half extent	0
chassis_friction	Body contact friction, 0..2	0.3
center_of_mass	Chassis-local mass centre, metres	[0,0,0]
inertia_half_extents	Box dimensions used for mass distribution, independent of contact shape	null: use half_extents

Rounding stays inside half_extents. Shorten low front/rear overhangs so wheel contact can lift the chassis before the body catches the ramp. Keep the collider large enough to protect the cockpit; the visual model does not define collision. Inertia dimensions let a shorter collision shape retain stable pitching/rolling behaviour. A lower centre of mass helps resist nose-diving under braking. These fields require respawning. The example passes headless 20/30/40-degree incline tests plus braking and steering tests; this does not guarantee every map seam or vertical ledge is traversable. Wheels remain raycasts, and vertical walls remain obstacles. No teleporting or artificial ramp boost is used.

Engine sound

Opt in per definition:

"engine_audio": { "enabled": true, "volume": 0.45, "idle_pitch": 0.7, "max_pitch": 2.8 }

The host synthesizes an original layered exhaust pulse with filtered noise. No downloaded sound asset or extra file is required. It plays for the occupied vehicle; parked engines are silent. Pitch follows speed and absolute throttle, including reverse and free revving while stopped. Full throttle increases volume; releasing it smoothly drops the engine back towards idle. Pitch/volume changes are smoothed, Escape/replay fades it silent, and exiting or disabling the mod fades/removes the voice. This is a driver-focused mono sound, not a spatial multi-car mixer or a simulated gearbox. There is currently no custom sample slot.

Volume is 0..1, idle_pitch 0.25..2, max_pitch idle_pitch..5 (multipliers of the 80 Hz synth fundamental). All are validated. sdk.vehicle.tune(key,{engine_volume=0.5}) changes volume live; use zero to mute. Mario Kart exposes this in its mod settings.

Authoring and reuse

See Mixamo vehicle animation workflow for the full sequence, calibration, Blender fitting, native export and packaging commands. The host's animation slots, steering blend, stance hand-offs, board hiding and shared motion interpolation apply automatically to every vehicle definition. The example fitting scripts contain kart geometry targets; adjust those targets for another vehicle without changing the host.